For many decades, masonry houses were built with marble or limestone used for the window and door sills, lintels, belt courses, and outdoor steps. These stone types were also used, in grander houses, for quoins at the corners and modillions in the cornice. The stones are dirtied and eventually damaged by atmospheric pollutants, bird droppings, and even the dirt carried across steps by our shoes. It’s wise to spend some time carefully cleaning limestone and marble, before damage can occur.
Consider that marble, a metamorphic stone, is created when sedimentary limestone is subjected to high pressure and heat underground. The calcite-based stones are closely related, and both are damaged in the presence of acids—whether lemon juice on the kitchen counter or acid rain on a limestone lintel. They benefit from similar cleaning techniques.
Causing further damage
Many cleansers will “do the trick”—that is, remove the dirt and leave the sill or steps looking cleaner. Depending on the makeup of the cleaning agent, though, short-term improvement may come at a cost. Common acidic cleansers include mineral-deposit and rust-stain removers, and bathroom cleaners. Acids dissolve the calcite-based structure of marble and limestone, leaving the surface rougher and therefore more likely to pick up soil. Continued use of the agent, as well as pollutants, dissolve the stone, causing pitting and loss of detail.
Avoid acidic cleaners
Keep in mind that marble and limestone are vulnerable to dissolution in acids. This is the reason that acid rain is so damaging to marble statuary and cemetery monuments. Make certain that the detergent you use is classified as non-ionic, or that its pH is slightly alkaline. Look for cleaners designed for marble countertops, etc., and follow the directions on the container.